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Homework Help: Rotary Motion

  1. Feb 20, 2008 #1
    1. The problem statement, all variables and given/known data
    A 1.3 kg block is tied to a string that is wrapped around the rim of a pulley of radius 7.2 cm. The block is released from rest.

    (a) Assuming the pulley is a uniform disk with a mass of 0.30 kg, find the speed of the block after it has fallen through a height of 0.41 m.

    (b) If a small lead weight is attached near the rim of the pulley and this experiment is repeated, will the speed of the block increase, decrease, or stay the same? Explain.

    2. Relevant equations

    3. The attempt at a solution
    Since the block was just falling I assumed you could just use the 3rd kinematics and solve for vf with vi = 0. Substituting the variables I got Vf = sqrt (2 x 9.81m/s^2 x .41m) and the answer came out to be 2.836m/s. Any reason why this wouldn't work?
  2. jcsd
  3. Feb 20, 2008 #2


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    Homework Helper

    You can't assume a=9.81 m/s^2, though. If the block is attached to the pulley by a string, don't you think that might change its acceleration?
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